Implantable cardioverter-defibrillator (ICD) art has long distinguished ventricular arrhythmias (VAs) by rate and type. Ventricular Tachycardias (VTs) generally are those arrhythmias with rates between 150 and 250 bpm. These rhythms can be further differentiated by their ECG configuration as either monomorphic or polymorphic. Arrhythmias with rates above the upper VT range are typically classified as Ventricular Fibrillation (VF).
To treat each type of arrhythmia in the appropriate manner, some ICDs are equipped with “tiered therapies” for delivering therapy based on the type of arrhythmia detected by the device. Such devices, often referred to as Implantable Cardioverter-Defibrillators (ICDs), generally differentiate arrhythmias by rate. For example, less dangerous arrhythmias such as VTs may be treated by delivering a series of low-power, relatively high-rate, pacing pulses to the heart. This therapy is often referred to as anti-tachyarrhythmia pacing therapy (ATP). In contrast, more perilous arrhythmias such as VFs are often treated using a more aggressive shock therapy. For example, many ICDs may be programmed to first treat a VT with low-power ATP. If ATP fails to terminate the VT or the VT progresses to ventricular fibrillation, the device may then deliver one or more high-power cardioversion or defibrillation shocks.
As may be apparent from the foregoing discussion, some types of arrhythmias are better treated with high-voltage shocks than ATP therapy. For this reason, high-voltage shocks may be “over-used” to treat conditions that could otherwise be successfully treated using ATP. The more aggressive treatment is prescribed because most clinicians prefer a fast termination of the arrhythmia rather than allowing time to pass while a determination is made as to whether ATP therapy will terminate the rhythm. This tendency to utilize a more aggressive therapy is intended to reduce the possibility of patient syncope. However, this rationale results in some patients unnecessarily enduring the pain of a high-voltage shock when painless ATP therapy could have successfully terminated the rhythm. A good example of this over-aggressive therapy selection involves monomorphic VTs having rates above 200 bpm. These VTs are often treated by high-energy shocks when, in fact, they may generally be terminated by low-energy ATP therapy.
Preventing the unnecessary delivery of high-voltage shocks has long been recognized as a very desirable goal. As a result, monitoring the rhythm during the charging of the high-voltage capacitors in preparation for shock delivery has been proposed. For example in U.S. Pat. No. 4,949,719, issued to Pless et al, and U.S. Pat. No. 5,191,884 issued to Gilli et al, the implanted device monitors heart rhythm during charging to determine whether the arrhythmia has spontaneously terminated, and thereafter aborts the charging of the output capacitors if the rhythm has returned to normal.
Another approach to this issue is found in U.S. Pat. No. 5,318,591, issued to Causey et al., and incorporated herein by reference in its totality. The '591 patent teaches a three-tiered progressive therapy approach using ATP as a first recourse, followed by delivery of a cardioversion pulse in the event ATP failed, with a defibrillation shock to be delivered if cardioversion is unsuccessful. The ICD begins charging its high-powered capacitors in parallel with the application of the ATP therapy. In addition, this charging may also start in parallel with the verification interval immediately following the previous therapy, during which time the ICD attempts to verify arrhythmia termination.
Numerous other patents describe ATP pacing including U.S. Pat. No. 5,193,536, issued to Mehra, U.S. Pat. No. 5,458,619 issued to Olson, U.S. Pat. No. 6,167,308, issued to DeGroot, and U.S. Pat. No. 6,178,350, issued to Olson, et al. Other patents describe in more detail systems that analyze the sequence and timing of events prior to the selection of a therapy. Such patents include U.S. Pat. No. 5,205,283 issued to Olson, U.S. Pat. No. 5,193,550 issued to Duffin, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz at al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al.
In the patents listed above, several basic strategies are generally followed. A first strategy is to associate each type of arrhythmia with a predetermined set of criteria. Next, a patient's heart rhythm is monitored to identify a heart event, including intervals and/or rates associated with the event. This information is then compared against the various criteria sets to analyze the likelihood that the event may be characterized as a specific type of arrhythmia. Monitoring continues until one of the criteria sets is met, resulting in detection and diagnosis of the arrhythmia. A second basic strategy involves defining a set of criteria for events, intervals, and rates that is generally indicative of a group of arrhythmias. After the criterion is met, the preceding and/or subsequent events are analyzed to determine which specific arrhythmia is present.
As is evident from a review of the above-cited references, many implantable anti-tachycardia pacemakers provide a variety of ATP regimens. Normally, these regimens are applied according to a pre-programmed sequence, such as burst or ramp therapies among others. Each therapy extends over a predetermined number of pacing pulses. After delivery of these pacing pulses, the devices generally determine whether the pulses were effective in terminating the detected arrhythmia episode as may be confirmed by a return to sinus rhythm. This is identified by detecting a sequence of spontaneous depolarizations separated by greater than a predefined interval. In the absence of detected termination, the ICD applies more aggressive therapies such as synchronized cardioversion pulses or defibrillation shocks. While the delivery of ATP in some cases makes shock therapy unnecessary, a further reduction in the delivery of high-voltage shocks is still desirable.